It sounds to me like he has the HUD turned off. The debuffs that are active on his character are shown at the upper left of the HUD just under his character's name. The also appear in his unit frame in the Player, Party and Raid10 unit frame panels.

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What people don't get is that I am, ultimately, an artist at heart. My brush has two colors, 1 and 0, and my canvas is made of silicon.